The present invention relates to an improvement in a winding spring for a seat belt winding apparatus.
Ordinarily, a seat belt winding apparatus comprises an output drum, a take-up drum, a winding spring stretched and wound in the S-shaped form between said output drum and take-up drum, a belt winding drum disposed coaxially with said output drum and a webbing having one end fixed to said belt winding drum and wound on said belt winding drum. One end of the winding spring is fixed to the output drum, but the winding spring is set free to the take-up drum and such a curling property that the winding spring is naturally wound on the take-up drum to wind the webbing on the winding drum in the normal state, that is, a natural radius of curvature, is given to the winding spring. Accordingly, if the webbing is pulled out, the output drum is rotated with rotation of the winding drum and the spring is wound on the output drum from the take-up drum. When the pull-out force on the webbing is released, the winding spring is wound back on the take-up drum from the output drum to wind the webbing on the winding drum. In order not to impart a compressive force to a wearer when the webbing is pulled out and attached to the wearer, in conventional winding springs, the natural radius of curvature is gradually increased in the direction of or toward the take-up drum along the entire length of the spring proportionally to the pull-put length of the webbing, and negative spring characteristics varying linearly along the entire length are given to the spring. It will readily be understood that in such winding spring, when the webbing is pulled out completely to the final end thereof, rotation is caused in the take-up drum by inertia to loosen the surplus portion of the spring wound on the take-up drum. As a result, there is caused a phenomenon that the spring rises from the take-up drum. When this phenomenon takes place, the stretched portion of the spring where the return force is small becomes engaged with the inner face of a casing covering the output drum and take-up drum, and as a result, an operation trouble is caused to render winding of the webbing impossible.
A winding spring in which occurrence of the above-mentioned operation trouble is prevented was proposed by applicant in published Japanese Patent Application No. 55921/1979. According to this previously proposed invention, the natural radius of curvature given to the winding spring is gradually increased in the direction of or toward the take-up drum in the normal use region of the spring while the natural radius of curvature in the surplus use region is gradually decreased in the direction of or toward the take-up drum, and the return force of the spring to the take-up drum (the torque of the output drum) as one spring characteristic is gradually decreased in the normal use region and gradually increased in the surplus use region. In the winding spring having this structure, even if the webbing is abruptly pulled out to the final end at a speed of, for example, about 8 m/sec carelessly, since a sufficient winding force, that is, a holding force, preventing rotation of the take-up drum by inertia is given to the surplus region of the spring wound on the take-up drum, the above-mentioned operation trouble is not caused at all. However, if the webbing is pulled out suddenly to the final end thereof at such a high speed as exceeding 8 m/sec, though such seldom takes place, the take-up drum is rotated by inertia against the winding force of the spring to the take-up drum, and the spring is loosened around the take-up drum in the same manner as described above and becomes engaged with the inner face of the casing, resulting in occurrence of the above-mentioned operation trouble. The inventor made researches with a view to clarifying why such undesirable phenomenon takes place, and as a result, it was found that since the surplus region of the spring on the take-up drum holds the take-up drum too strongly, the free end of the spring is caught on the drum surface and when the take-up drum is turned by inertia, the free end is rotated together with the take-up drum to loosen the portion of the spring wound on the take-up drum and causes the above-mentioned undesirable phenomenon. Occurrence of this phenomenon is especially conspicuous when a burr directed to the take-up drum is formed on the cut edge of the free end of the spring at the shear processing step.